In an effort to protect vehicle operators and occupants, vehicle manufacturers and their suppliers continue to direct their efforts to the design and manufacture of vehicles with additional and improved safety features. One such safety feature that has been incorporated into a number of vehicle types is Anti-lock Braking Systems (ABS) which can take a variety of forms.
Such ABS systems are generally designed to maximize the ability of a vehicle operator to bring a vehicle to a controlled stop on any type of road surface. The system accomplishes this goal by preventing the vehicle brakes from prematurely halting vehicle wheel rotation, or "locking" the vehicle wheels, regardless of the road surface and the pressure applied to the brake pedal by the vehicle operator.
Typical vehicle anti-lock brake systems comprise a vehicle wheel speed sensor for providing input to an anti-lock brake system control unit. The control unit controls an anti-lock brake system control valve interposed between the brake master cylinder and the individual wheel brake of a hydraulic brake circuit. The control valve, in turn, regulates hydraulic brake fluid pressure in the individual wheel brakes so as to implement anti-lock braking.
In most systems, raw wheel speed is low pass filtered to reduce spurious noise. Generally, such filtering is accomplished through the use of a first order filter with the result stored. The filtered wheel speed is referred to by those skilled in the art as FVEL. Differentiation and further filtering yields wheel deceleration and wheel jerk.
From a control velocity, wheel behavior is characterized by higher order derivatives. The determined signals of filtered wheel speed (FVEL), filtered wheel deceleration and wheel jerk may be used to detect the onset of instability. (departure) so as to effect washboard surface rejection and to further identify wheel recovery.
In operation, the vehicle wheel speed sensor not only measures the vehicle wheel speed, but also provides input to the control unit for determining an estimated reference speed (VREF) for the vehicle at the corresponding wheel and ABS channel. In normal operation, the wheel speed references are decayed so as to ensure maximum braking efficiency while providing a reference for slip detection.
The wheel speed references (VREFs) for each of the control channels are used to calculate the average vehicle speed (AVEL). Different proportioning is, of course, used in the averaging calculation between the front and rear wheel speed references so as to optimize the accuracy of the estimated vehicle speed reference. The resultant estimated vehicle speed reference is used for system control and is also required for the calculation of vehicle deceleration.
The control unit monitors the vehicle and vehicle wheel speeds for an indication of an anti-lock braking event. First, based upon the vehicle speed, the control unit typically determines a slip threshold. Using the vehicle velocity as a reference, slip threshold may be expressed as the difference between a selected velocity at a wheel and the vehicle velocity (AVEL).
Next, the control unit compares the vehicle wheel velocity to the vehicle velocity to determine a departure depth. Again, using the vehicle velocity as a reference, departure depth may be expressed as the difference between the vehicle velocity and the wheel velocity. During normal vehicle braking, the wheel velocity closely matches the vehicle velocity. Thus, during normal vehicle braking, the difference between the vehicle velocity and the wheel velocity is nominal.
During an anti-lock braking event, however, the wheel velocity decreases significantly below, or "departs" from the corresponding vehicle reference velocity. In such a situation, as for example during hard braking and on ice-covered roads, the frictional force between the vehicle brake pads and the vehicle brake drum/disc exceeds that between the vehicle wheel and the road surface. Uncontrolled, such a frictional force differential causes the vehicle wheel to cease rotating, or to "lock".
In turn, locking causes the vehicle wheels to slip or "skid", rather than roll over the road surface. Such vehicle wheel skidding dramatically reduces traction and the ability of the vehicle operator to bring the vehicle to a controlled stop.
To prevent such vehicle wheel lock in the accompanying problems, the control unit of an anti-lock brake system activates the anti-lock brake system control valve to regulate the hydraulic brake fluid pressure in the individual wheel brakes during an anti-lock braking event. More specifically, the control unit compares the departure depth to the slip threshold and activates the control valve when the departure depth exceeds the slip threshold. This is done in order to isolate the individual vehicle wheel brakes in the hydraulic brake circuit from the master cylinder, thereby halting any increase in brake fluid pressure in the vehicle wheel brakes and preventing incipient vehicle wheel lock.
More particularly, when, during vehicle braking, the departure depth exceeds the slip threshold, the control valve isolates brake fluid in the individual wheel brakes from the increasing brake fluid pressure in the master cylinder in order to hold brake fluid pressure in the wheel brake constant. If the isolated brake fluid pressure applied to the wheel brakes is still high enough to cause incipient wheel lock, the anti-lock brake system then bleeds, or dumps, brake fluid from the wheel brake to reduce brake fluid pressure therein.
Thereafter, the anti-lock brake system typically holds brake fluid pressure in the wheel brake constant until such time as the departure depth no longer exceeds the slip threshold, indicating the vehicle wheel is again traveling at or near the velocity of the vehicle. At that time, the anti-lock brake system then increases, or builds, brake fluid pressure in the wheel brake by reapplying brake fluid thereto. Reapplication of brake fluid to the wheel brake may be at a steep or gradual rate, or some combination thereof, depending upon the circumstances or the control desired.
One problem associated with the above-described prior art anti-lock brake systems is the possibility for false, or premature activation. Premature activation can occur in a number of circumstances, such as where a road surface is partially ice-covered or bumpy and individual vehicle wheels experience different coefficients of friction during braking. Typical anti-lock brake systems activate when the departure depth of any one of the vehicle wheels exceeds the slip threshold, despite the fact that the wheel would not have experienced excessive slip.
Moreover, typical anti-lock brake systems decrease the slip threshold after activation in order to increase anti-lock brake system sensitivity during an anti-lock braking event. Under normal conditions, this action improves the ability of the vehicle operator to slow or stop the vehicle in a controlled fashion. However, in the event of premature activation of the anti-lock brake system, such activation merely compounds the problem of the Anti-lock brake System.
In addition to the above-noted problems, undesirable results can also be obtained when one or more of the vehicle wheels are determined to be in spin-up, i.e., if the actual vehicle speed is lower than the wheel speed and the estimated reference speed (VREF). More particularly, if one wheel is in spin-up, a compare circuit which is comparing values from other wheels will indicate that an increase in reference speed is desired. However, when the spin-up goes away on the wheel in question, slip will be indicated on all wheels and the anti-lock braking system will be required to isolate and dump off all the pressure. This is, of course, an undesirable event.
So-called "spin-up" protection has been used in prior art anti-lock braking systems and has heretofore normally been triggered at a single predetermined acceleration, regardless of vehicle speed. Thus, for all AVEL, if the rate of change in wheel speed (acceleration) of a given wheel exceeded the threshold, spin-up protection was triggered. The vehicle speed reference value was then adjusted accordingly or, at a minimum, was determined to be in error.
While the prior art spin-up protection technique is a marked improvement, it does not provide accurate reference information at all estimated vehicle speeds, i.e., the possible acceleration of a vehicle is not a constant across the possible range of vehicle speeds.
Consequently, a need has developed for an improved method of estimating a vehicle speed reference value (VREF) which more closely tracks the relationship between estimated vehicle speed and wheel acceleration so as to prevent false slip detection and premature ABS activation.